Insulated coating for silicon steel sheet



INSULATED COATING FOR SILICON STEEL SHEET Kamematsu Matsuda, TobataCity,

Yawata Iron and Steel Co., poration of Japan Japan, assignor to Ltd.,Tokyo, Japan, a cor- The present invention relates to the method ofproducmg an interlamellar insulated coating on silicon steel sheet orstrip.

With the recent development of the quality of silicon steel sheet orstrip, it is very frequent to anneal it in a hydrogen atmosphere at thetemperature of 800 C. for the purpose of removing strain after theworking steps, such as, shearing, punching, and stamping. In such cases,insulated films coated on silicon steel sheet or strip of the prior art,e.g., varnish-coated films, and iron oxide coated films have been flakedofif, carburized, and deteriorated, so that the effect of insulation hasbeen reduced.

Recently the adoption of phosphate coating has been introduced toprovide insulation, but resulted in a considerable reduction ofinsulation after an anneal in an atmosphere containing a substantialamount of hydrogen.

It is one object of my invention to provide a firmly adherent coating ofinsulation on silicon steel sheet or strip.

It is another object of the invention to provide a new and novel methodof producing a strong adherent insulated coating on silicon steel sheetor strip.

It is another object of the invention to provide a firmly adherentinsulated coating on silicon steel sheet or strip with a superiorproperty for interlamellar resistance and space factor.

It is another object of the invention to provide an excellent insulatedcoating on silicon steel sheet or strip which is never attacked by thetransformer oil, nor affects the transformer oil badly.

It is an additional object of the invention to provide a superiorinsulated coating on silicon steel sheet or strip with an excellentresistance to corrosion and scratch or multilation.

With the above and other objects in view that will hereinafter appear,the nature of the invention will be more fully understood by followingthe detailed description and the appended claims.

The insulated film of my invention is produced by coating an aqueoussolution containing 10-60 Weight percent of zinc bichromate on thesurface of silicon steel sheet or strip with a thin and uniform coatingthereon, and heating the thus coated silicon steel sheet or strip at atemperature between 500 and 700 C. for a period of ten to forty seconds.The insulated film of my invention is also formed by heating siliconsteel sheet or strip coated with an aqueous solution of the samecomposition added with 1-4% of glycerine at a temperature of ZOO-500 C.Moreover, 2-5% of boric acid may be added to the aqueous solution with aview to making the insulated film glossy and scratch-proof.

The insulated film may be produced in any other range than theabove-mentioned concentration of zinc bichromate. However, in theconcentration below 10% of Zinc bichromate, the coated film becomes verythin, which results in a poor insulation property, and the film isproduced in excessive thickness if the aqueous solution nited StatesPatent contains more than 60% of zinc bichromate. Accordingly, I limitthe concentration range of 10-60% of zincbichromate in my invention.

It is necessary to heat silicon steel sheet or strip coated with thesolution of the invention above the temperature of 500 C. when thesolution does not contain glycerine. Under the temperature of 500 C.,the incomplete decomposition of zinc bichromate and the subsequentinsufiicient reaction on the surface of silicon steel sheet or stripwill result in a hydrous film, consequently the film absorbs moisturegradually in the course of a prolonged exposure in the air after theformation of the film, which causes a considerably reduced insulation.Accordingly, it is required to heat silicon steel sheet or strip coatedwith the aqueous solution above the temperature of 500 0, preferablyabove the temperature of 600 C. in order to develop the desired results.

With the coating solution added with 15% of glycerine, the heatingtemperature of the base material at a temperature of 200-500 C. issuflicient, because the solution adheres tightly to the base sheet orstrip with the reaction of zinc bichromate and glycerine at a lowertempertaure and with the subsequent solidification of the thus reacted.product. However, it does not matter to heat above the temperature of500 C. if it is required to do so. As the film produced by coating anaqueous solution of zinc bichromate with or without an addition ofglycerine. is rough and dull in appearance, and easily mutilated, boricacid may be added in the range of 25% in order to develop a glossy andsmooth surface on the silicon steel sheet or strip.

An embodiment of the invention is described hereinafter. A mixture of8.14 kg of zinc oxide and 20 kg of anhydrous chromate is dissolved inWater, stirred, and reacted together. The reaction is as follows:

In this reaction, the equivalents of zinc oxide and anhydrous chromate,respectively, must be added. If the amount of CrO is in excess, the filmtends to be hydrous, on the contrary, the film tends to be thick due tothe remains of ZnO if a litttle amount of CrO is present.

The aqueous solution thus produced may be employed as it is, andbesides, as described hereinbefore, the addition of glycerine and boricacid is effected as follows: 4 kg. of boric acid is dissolved andstirred in water, then litres of an aqueous solution is produced byadding more water thereto, and the solution is cooled to the roomtemperature, finally 2 kg. of glycerine is added thereto.

The aqueous solution thus produced is uniformly coated on the surface ofsilicon steel sheet or strip by a conventional manner. Removal of oxidespresent on the surface of silicon steel sheet or strip is not requiredby means of pickling before the base sheet or strip is coated with thesolution of my invention.

In the course of the production of silicon steel sheet or strip, variousoxides and silicates, e.g., silica, iron oxide, calcium silicate, andmagnesium oxide, are formed on the surface of silicon steel sheet orstrip. The completeelimination of such oxides and silicates is notrequired for the formation of the insulated film in accordance with theinvention. As the higher insulation efficiency is acquired in case ofsilicon steel sheet or strip with oxides present on the surface thereofthan in the base sheet or strip without oxides, the concentration of theaqueous solution and the amount of coating may be determined by takingthe condition of the surface of silicon steel sheet or strip intoconsideration.

The silicon steel sheet or strip thus coated with the aqueous solutionwith no glycerine is heated at a temperature of 500-700" C. for aperiodof 10-40 seconds. At higher temperatures, a heating process is finishedin a shorter period of time. The heating time depends on the amount ofcoating. 'It is preferable to heat silicon steelsheet or strip coatedwith the aqueous solution of 30% zinc bichromate with the amount of'25cc. per

square meter at the temperature of 600 C. for a period of about twentyseconds. I p v 7 With the coating solution added with glycerine, caremust be taken to the heating speed. Gradual heating is necessary,because the film will be peeled off owing to an abrupt heating. Theheating speed must be determined according to the concentration of thecoating solution and the amountof coating to be applied, but anappropriate speed of heating can be readily found bythose skilled in theart. 1 The insulated film thus produced is regarded as the coated filmcomprising complicated chromates consisting of chromium-oxide, iron, andzinc, and in some cases,

silicates. v i

. The film will not be flaked oii by being bent at an angle of 180 withthe radius of'25 millimeters. An anneal in the hydrogen at thetemperature of 800 C. after the formation of the film results in anegligible reduction of insulation resistance, thereby nothing isaffected to the practical application. I

The following table represents the comparative results.

Interlamellar resistance of film As described hereinbefore, corrosion ofsilicon steel sheet or strip is prevented eifectively by the coated filmof my invention. The transformer oil will not attack the film, andbesides, the film will not deteriorate the transformer oil. Moreover,the formation of the film does not destroy the magnetic property ofsilicon steel sheet or strip.

From the foregoing description, it is thought that the novel details ofinsulated coating, the manner of coating, and the advantages of theinvention, will be apparent to those skilled in the art to which itrelates.

It is of course to be understood that the details of coating compositionand method may be variously changed and modified without departing fromthe spirit and scope of my invention as set forth in the appendedclaims.

Having thus described the invention, what I claim as new and desire tosecure by Letters Patent, is:

1. A method of producing an insulated coating on the surface of siliconsteel sheet or strip which comprises coating an aqueous solution of10-60 weight percent of zinc bichromate in the form of a thin anduniform film on the surface of the silicon steel, and heating the thuscoated silicon steel at a temperature of 500-700 C. for a period of10-40 seconds, thereby a firmly adherent film of insulation is producedon said silicon steel sheet or strip.

2. A method of producing an insulating coating on the surface of siliconsteel which comprises coating said surface with a thin and uniform filmof an aqueous solution consisting essentially of from 10 to 60 percentby weight of zinc bichromate, from to 5 percent by weight of boric acidand from 1 to 5 percent by weight of glycerine, and heating the thuscoated silicon steel at a temperature of from 200 to 500 C. for a periodof from to 40 seconds, thereby forming a firmly adherent film ofinsulation on the silicon steel.

3. A method of producing an insulating coating on the surface of siliconsteel which comprises coating said surface witha thin and uniform filmof an aqueous solution consisting essentially of from 10'to 60 percentby weight of zinc bichromate and from 2 to 5 percent by weight of boricacid and heating the thus coated silicon steel at a consistingessentially of from 10 to 60 percent by weight of zinc bichromate andfrom l to 5 percent by weight of glycerine and heating the thus coatedsilicon steel at a temperature of from 200 to 500 C. for a period offrom 10 to 40'seconds, thereby forming a firmly adherent film ofinsulation on the silicon steel.

5. A method of producing an insulating coating on the surface of siliconsteel which comprises coating said surface with a thin and uniform filmof an aqueous solution consisting essentially of from 10 to 60 percentby weight of zinc bichromate, from 2 to 5 percent by weight of boricacid and from 1 to 5 percent by weight of glycerin, and heating the thuscoated silicon steel at a temperature of from 200 to 500 C. for a periodof from 10 to 40 seconds,-thereby forming a firmly adherent film ofinsulation on the silicon steel.

6. A method of producing an insulating coating on the surface of siliconsteel which comprises coating said surface with a thin and uniform filmof an aqueous solution consisting essentially of from 10 to 60 percentby weight of zinc bichromate, from 0 to 5 percent by weight of boricacid and from 0 to 5 percent by weight of glycerine, and heating thethus coated silicon steel at a temperature of from 500 to 700 C. for aperiod of from 10 to 40 seconds, thereby forming a firmly adherent filmof insulation on the silicon steel.

References Cited in the file of this patent UNITED STATES PATENTS1,147,971 Perry July 27, 1915 2,210,850 Curtin Aug. 6, 1940 2,472,592Kiefer June 7, 1949 2,618,578 Kreml Nov. 18, 1952 2,788,292 Giesker etal. Apr. 9, 1957 FOREIGN PATENTS Great Britain Jan. 4, 1888

1. A METHOD FO PRODUCING AN INSULATED COATING ON THE SURFACE OF SILICON STEEL SHEET OR STRIP WHICH COMPRISES COATING AN AQUEOUS SOLUTION OF 10-60 WEIGHT PERCENT OF ZINC BICHROMATE IN THE FORM OF A THIN AND UNIFORM FILM ON THE SURFACE OF THE SILICON STEEL, AND HEATING THE THUS COATED SILICON STEEL AT A TEMPERATURE OF 500*-700*C. FOR A PERIOD OF 10-40 SECONDS, THEREBY A FIRMLY ADHERENT FILM OF INSULATION IS PRODUCED ON SAID SILICON STEEL SHEET OR STRIP. 